1. Field of the Invention
The present invention relates to conductive powder to be used for formation of a circuit of a wiring board, formation of a shield layer, formation of an electrode of electronic parts, formation of an electrode with solder, a conductive adhesive, a thermal conductive adhesive, etc., and a method for preparing the same.
2. Prior Art
As a method for forming a conductive circuit on a printed wiring board, there is a method in which conductive powder such as gold, silver, copper and carbon is used and mixed with a binder, an organic solvent and, an additive(s), if necessary, to prepare a paste state material and the material is applied to the board (see, for example, “Electronic Material”, pp. 42-46, No. 10, 1994). In particular, in the field in which high conductivity is required, gold powder, silver powder, palladium powder or alloy powder thereof has been generally used.
Of these, a conductive paste containing silver powder has good conductivity, so that it has been used as a wiring layer (a conductive layer) for a printed wiring board, electronic parts, etc., or for forming an electric circuit or electrodes of electronic parts. However, when an electric field is applied thereto under atmosphere of high temperature and high humidity, electrodeposition of silver, which is so-called migration, occurs in the electric circuit or electrodes whereby there is a defect that short circuit between electrodes or wirings occur. To prevent from causing the migration, there have been investigated some measures, for example, a moistureproof coating is applied onto the surface of the conductive material or a corrosion preventive agent such as a nitrogen-containing compound is added to a conductive paste, but these measures are insufficient to obtain an effect. When a silver-palladium alloy powder is used in place of silver powder, migration resistance can be heightened, but silver and palladium are expensive, so that there is a defect that expensive silver-palladium alloy powder must be used.
Also, to obtain a conductive material having good conductive resistance, an amount of silver powder to be formulated must be increased but the silver powder is expensive, so that there is a defect that the conductive paste is also expensive. When silver-plated copper powder is used, migration can be prevented and an inexpensive conductive paste can be obtained. However, when the surface of the copper powder is coated with silver uniformly and thick, an improved effect in preventing from migration is not sufficient. Moreover, when soldering is carried out to a conductive paste using silver powder, there is a defect that sufficient conjugation cannot be carried out since silver is alloyed with the solder which is a phenomenon that silver is dissipated and dissolved in the solder to disappear.
On the other hand, there is a case wherein copper powder is used in addition to the silver powder. However, a conductive paste using the copper powder has an easily oxidizable property of copper after heat-curing, and oxygen contained in air and in a binder and the copper powder react to form an oxidized film on the surface thereof whereby the conductivity is markedly lowered. Thus, there is disclosed a copper paste to which various kinds of reducing agent(s) are added to prevent from oxidation on the surface of the copper powder to stabilize the conductivity. However, an effect to stabilize the conductivity does not cover the silver powder, and an electric resistance is increased in a high temperature and high humidity test, whereby there is a defect that the conductive circuit becomes a short-circuited state.
In the prior art, when the conventionally known conductive paste is used as an adhesive, it has a drawback that the conductive paste is expensive since the conductive powder is expensive as compared to a solder paste. Accordingly, it has been earnestly desired to develop a conductive adhesive that has higher reliability in conductive property than that of the copper paste, excellent migration resistance than the silver paste and has the same operatability in drying and curing as that of the solder paste.
Also, in an adhesive required to have thermal conductivity, a filler having good thermal conductivity such as powder of silver, copper, boron nitride, etc. must be mixed with a higher filling ratio, but if a formulation amount of the powder is increased, a viscosity thereof is also increased and fluidity thereof becomes poor, so that there is a defect that production and use of the paste become difficult.
As a method for forming a conductive circuit using a conductive paste, a conductive paste in which conductive powder is dispersed in a binder to make a paste state material is coated on the surface of a substrate (a board) 3 or filled with a through hole 4 as shown in FIG. 1 to form a conductive layer 1. Incidentally, the reference numeral 2 in FIG. 1 is a copper foil and 5 is an insulating layer.
Also, as another method for forming a conductive layer in a through hole formed in a printed wiring board, there is a method in which copper plating is applied on an inner surface of the through hole to form a conductive layer.
In general, when an interlayer connection is carried out by filling a conductive paste for filling a hole in a through hole, it requires high conductivity whereas it is a small hole, so that the conductive paste is filled in the hole as much as possible and the conductive paste must be buried therein without gap. Thus, the conventional conductive paste for filling a hole is required to have a high ratio of the conductive powder, but when the ratio of the conductive powder is made high, a viscosity of the conductive paste becomes high and a filling property to the hole becomes poor. On the other hand, when a ratio of the binder is made high, a viscosity of the same becomes low and a filling property to the hole is improved, but there is a defect that conductivity is lowered.
When a packing density of conductive powder is heightened, a viscosity of the conductive paste in which the conductive powder is formulated with a high ratio can be made low than in the case in which conductive powder having a low packing density is used, but in the prior art techniques, it was difficult to heighten its packing density. In particular, in the case of silver powder having good conductivity, silver is soft in hardness and when agglomerated powder is subjected to disagglomeration, deformation in shape of the powder occurs accompanying with the operation, so that it was difficult to obtain conductive powder having high packing density. A packing density of commercially available silver powder is generally about 55% by volume in a relative value, and at most 65% by volume or so, and thus, it is extremely difficult to obtain conductive powder having 68% by volume or more stably. In the present specification, the relative packing density by volume is calculated by dividing a tapping density of powder by a theoretical density of the powder.